1. Field of the Invention
The present invention relates to a pressure-sensitive adhesive tape, in particular, to an adhesive tape in which a pressure-sensitive adhesive layer is formed on a foam backing tape.
2. Description of the Prior Art
Conventional single-coated and double-coated adhesive tapes in which an adhesive material is coated on a backing have been widely used in a variety of manners such as packaging, masking, jointing of electrical insulating materials and the like. However, when jointing structural materials made of metal, plastic, wood, gum or the like, especially the rough or irregular surfaces of such materials, adhesive tapes in which pressure-sensitive adhesive material is coated on a foam backing are more advantageous, since elasticity of the foam enables the adhesive layer to adapt easily to the irregular surface.
For the foam backing, urethane foam sheeting, polyethylene foam sheeting, chloroprene rubber foam sheeting, foam sheeting made of synthetic resins such as polyvinyl chloride, polyviniyl acetate and the like have been commonly used. Nevertheless, these products have some disadvantages. Specifically, urethane foam backing is poor in resistance to weathering and the like. Similarly, with respect to polyethylene foam, polyvinyl chloride foam, polyvinyl acetate foam and the like, it is difficult to obtain a satisfactory material which sufficiently contains all the properties of stress relaxation, flexibility, extensibility and breaking strength. In particular, when polyvinyl chloride foam is employed as the backing, some problems are caused by the migration of plasticizer.
On the other hand, elastomeric gum adhesives and acrylic adhesives are commonly used for the pressure-sensitive adhesive which is coated on or impregnated into the backing. The gum adhesive primarily contains natural or synthetic rubber and is blended with tackifier resin and the like. On the other hand, the acrylic adhesives are obtained by copolymerization of a monomer mixture which contains an acrylate ester monomer as a main component and a polar monomer such as acrylic acid or the like. Of these materials, elastomeric gum adhesives are excellent in tackiness at low temperatures. Nevertheless, they are poor in resistance to weathering, heat and solvent attack. Therefore, acrylic adhesives have been used more popularly as an adhesive material for foam-backing tapes, except for some special cases.
Due to tendency in the mean value and distribution range of the molecular weight of the obtainable polymer molecules, the acrylic adhesive frequently lacks in cohesion. Therefore, in general manufacturing processes, the acrylic copolymer after copolymerization is further subjected to treatment with a crosslinking agent as which isocyanate compounds, epoxy compounds, polyamine compounds, melamine compounds, phenolic compounds, organic metal compounds and the like are well-known in the industrial field. By such a method, it is possible to improve the cohesive strength of the acrylic adhesives. However, even when the cohesive strength is raised by crosslinking treatment, tackiness at low temperatures and the initial tack of the acrylic adhesive deteriorates in accordance with increased crosslink density. On the contrary, if crosslink density is decreased, resistance to heat and solvent attack fails. Therefore, it is difficult to balance the various properties of the acrylic adhesive in a beneficial manner by regulation of crosslink density.
In other words, prior adhesive tapes have a problem to be solved in that all the properties of stress relaxation, flexibility, extensibility and breaking strength cannot be sufficiently imparted to the backing material at the same time, and also in that the pressure-sensitive adhesive material cannot demonstrate uniform excellence in cohesive strength, initial tack, tackiness at low temperatures and resistance to heat and solvent attack.